Rojo, who announced he would be leaving in an interview, will cost 20 million euros and Sporting will receive 20 percent of the profits if United sell him on for more than 23 million euros. He will become United's third major signing of the summer after Luke Shaw and Ander Herrera.

United confirmed the deal by tweeting: "£mufc has reached agreement with Sporting Lisbon to sign defender Marcos Rojo, subject to a medical and personal terms. A further announcement will be made when the process is complete.''

United turned to the Argentina international after accepting defeat in their attempts to sign Dortmund's Mats Hummels and Thomas Vermaelen, who joined Barcelona.

The Old Trafford club have been short of defenders after the summer departures of Rio Ferdinand, Nemanja Vidic and Patrice Evra and manager Louis van Gaal's decision to switch to a formation that requires three centre-backs.

The Dutchman had to give 20-year-old Tyler Blackett a debut in Saturday's defeat to Swansea but the left-sided Rojo is likely to take his place.

Nani came off the bench against Swansea but will be allowed to leave for the season to rejoin his former club. The winger has scored 40 goals in 228 games for United since leaving Sporting in 2007.

But while he was given a lucrative five-year deal last September, he has struggled in the last two seasons.